if you can get a cheap electric fridge I'd go for that every time - ok it may limit you a bit to piches with elec, but its so much nicer to have cold milk and butter that isn't melted - and it will propb be even hotter in France than here.
if you don't want to go elec I think you are best to get a cool box rather than a bag as Im sure they keep the cold better
Ah, so you take it off with you on your jaunts? Suspect we may be rather lazy beach based. If you don't drive about, does it loose heat faster than a 'normal' coolbox. Am debating the Coleman xtreme now, which does look rather cool <pun intended>